Problem: Write the equation for a parabola with a focus at $(9,0)$ and a directrix at $y=-4$. $y=$
The strategy A parabola is defined as the set of all points that are the same distance away from a point (the focus) and a line (the directrix). Let $(x,y)$ be a point on the parabola. Then the distance between $(x,y)$ and the focus, $(9,0)$, is equal to the distance between $(x,y)$ and the directrix, $y=-4$. Once we find these distances, we can equate them in order to derive the equation of our parabola. Finding the distances from $(x,y)$ to the focus and the directrix The distance between $(x,y)$ and $(9,0)$ is $\sqrt{(x-9)^2+y^2}$. [How did we find that?] Similarly, the distance between $(x,y)$ and the line $y=-4$ is $\sqrt{(y+4)^2}$. [How did we know that?] Deriving the formula by equating the distances $\begin{aligned} \sqrt{(y+4)^2} &= \sqrt{(x-9)^2+y^2} \\\\ (y+4)^2 &= (x-9)^2+y^2 \\\\ {y^2}+8y{+16} &= (x-9)^2{+y^2}\\\\ 8y&=(x-9)^2{-16} \\\\ y&=\dfrac{(x-9)^2}{8}-2\end{aligned}$ The answer The equation of our parabola is $y=\dfrac{(x-9)^2}{8}-2$. Here is the graph of our parabola. As expected, the distance between a point on the parabola, $(x,y)$, and the focus is the same as the distance between $(x,y)$ and the directrix. ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${10}$ ${11}$ ${12}$ ${13}$ ${14}$ ${15}$ ${16}$ ${17}$ ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ $y$ $x$ ${(x,y)}$